Polyisocyanate used as a raw material of polyurethane is industrially produced by reacting carbonyl chloride with polyamine for isocyanate reaction.
In this isocyanate reaction, corresponding polyisocyanate is produced from polyamine and hydrochloric gas is produced secondarily.
A production method for producing chlorine industrially by oxidizing the hydrochloric gas thus produced secondarily is known (cf. Patent Documents 1 and 2 listed below, for example).
A plant for manufacturing chemicals is provided with a gas treatment apparatus for detoxifying e.g. a harmful gas produced in a chemical process. This gas treatment apparatus comprises, for example, a filling column, a spray column, and a scrubber, and the one used for treating the harmful gas is sometimes called a detoxification column.
A detoxification column shown in FIG. 4 is known as an example of the gas treatment apparatus (cf. Patent Document 3 list below, for example).
This detoxification column 71 includes a treatment tank 72, a storage tank 73, and a pump 74. The treatment tank 72 has a gas-liquid contact chamber 75 in which a packed material is packed to improve efficiency of gas-liquid contact and also has showers 76 arranged over the gas-liquid contact chamber 75. A bottom of the treatment tank 72, the storage tank 73, the pump 74, and the showers 76 are connected via a circulation line 77.
A treatment liquid for detoxifying the harmful gas is stored in the storage tank 73. The treatment liquid is circulated in the following sequence: The treatment liquid is first pumped up by the pump 74 upward through the circulation line 77, then, sprayed from the showers 76 into the gas-liquid contact chamber 75 of the treatment tank 72. After passing through the gas-liquid contact chamber 75, the treatment liquid flows back to the storage tank 73 from the bottom of the treatment tank 72.
On the other hand, a harmful gas is supplied to the treatment tank 72 in such a manner as to flow upward from the bottom of the gas-liquid contact chamber 75 so that the harmful gas contacts the treatment liquid sprayed from the showers 76 in the vertically opposite direction for an effective gas-liquid contact, so that the harmful gas is detoxified. Then, the resultant gas is discharged from the treatment tank 72 to the atmosphere.
In Patent Document 3 listed below, a carbonyl-chloride-containing gas as the harmful gas is detoxified by this detoxification column using a sodium hydroxide liquid solution as the treatment liquid.    Document 1: Japanese Laid-open (Unexamined) Patent Publication No. Sho 62-275001,    Patent Document 2: Japanese Laid-open (Unexamined) Patent Publication No. 2000-272906, and    Patent Document 3: Japanese Laid-open (Unexamined) Patent Publication No. Hei 6-319946.